The present invention relates to an improved refractory material, a refractory comprising the refractory material and a method of producing the refractory. Particularly, the present invention relates to a magnesia-alumina type spinel clinker having improved cement coating and corrosion resistance properties in a rotary kiln for burning cement. More particularly, the present invention relates to a corrosion-resistant spinel magnesia refractory, particularly a refractory containing Fe.sub.2 O.sub.3 and TiO.sub.2, and a clinker used for producing the refractory.
Magnesia-chrome refractories are generally used as refractories for cement rotary kilns. Magnesia-chrome ores are natural ores and each have a complicated composition containing a spinel ore shown by Mg,Fe(Cr,Al,Fe).sub.2 O.sub.4 and impurity components such as SiO.sub.2, CaO and the like. A magnesia-alumina type spinel raw material having none of the disadvantages of magnesia-chrome refractories has recently been developed. Magnesia-alumina type spinel refractories produced by using the raw material are increasingly used as lining refractories in cement rotary kilns and other steel-making furnaces.
The durability of such magnesia-alumina type spinel refractories is generally improved by increasing the purity and density thereof or uniformizing the compositions thereof. However, since a deficiency of impurities at boundaries between magnesia and alumina grains causes the absence of inclusion for bonding between raw materials, the textures of such refractories are liable to loosen. When a brick is produced, therefore, the brick must be repeatedly compressed and burnt at a high temperature for increasing the density thereof. However, in this case, a satisfactory bonding state cannot be easily obtained, and the corrosion resistance is poor. When such refractories are applied to a rotary kiln for burning cement, since the cement does not sufficiently coat on and adheres to the inner wall of the kiln so as to protect the inner wall, the coating properties are poor, for example, and the field of application of the refractories is limited.
On the other hand, Japanese Patent Publication No. 60-34513 discloses a method of producing a spinel refractory having improved cement coating properties. This refractory is a spinel magnesia refractory formed by mixing 10 to 50% of spinel (MgAl.sub.2 O.sub.4) clinker and 50 to 90% of high-purity magnesia clinker, wherein 0.5 to 4.5% by weight of Fe.sub.2 O.sub.3 is mixed, or part or the whole high-purity magnesia clinker is substituted by a special magnesia clinker containing 3.0 to 5.0% of Fe.sub.2 O.sub.3 so that the Fe.sub.2 O.sub.3 content in a brick is 1.6 to 4.6%, thereby improving the cement coating properties.
Examples of methods of producing magnesia-alumina type spinel compositions are disclosed in Japanese Patent Laid-Open Nos. 2-30661 and 59-141461. The former publication discloses a method in which 0.5 to 8% by weight of magnesium titanate is contained as TiO.sub.2 in magnesia crystals or at boundaries between spinel grains so as to improve the corrosion resistance. The latter publication discloses a method of mixing 3 to 5% aluminum titanate with a ground spinel clinker in which 20 to 35% by weight of periclase is solid-dissolved, the mixture is molded and sintered, wherein aluminum titanate is preferentially solid-dissolved at boundaries between spinel crystal grains so as to improve the hot strength and spalling resistance.
Although the above conventional magnesia-alumina refractories were selected from refractories having characteristics suitable to the application conditions taking into account a balance between corrosion resistance and coating adhesion, these conventional refractories are still unsatisfactory in corrosion resistance at the firing zone and have some room for improvement.
Accordingly, it is an object of the present invention to provide a magnesia-alumina type spinel refractory having both corrosion resistance and coating adhesion in an attempt to broaden the range of application and improve the durability.